This disclosure pertains to microlithography (transfer of a pattern to a sensitive substrate), especially as performed using a charged particle beam. Microlithography is a key technology used in the fabrication of microelectronic devices such as integrated circuits, displays, and micromachines. More specifically, the disclosure pertains to charged-particle-beam (CPB) microlithography methods and apparatus in which certain alignment marks are detected so as to provide improved accuracy of lithographic exposure.
Microlithographic pattern transfer using a charged particle beam is regarded as highly accurate and capable of achieving very fine pattern-transfer resolution. However, charged-particle-beam (CPB) microlithography disadvantageously has low xe2x80x9cthroughputxe2x80x9d compared to optical microlithography (performed using deep UV light). (xe2x80x9cThroughputxe2x80x9d as used herein refers to the number of lithographic substrates, such as semiconductor wafers, that can be processed lithographically per unit time.) Various approaches have been investigated with the object of substantially improving throughput.
For example, several types of partial-pattern single-shot exposure systems (termed xe2x80x9ccell projection,xe2x80x9d xe2x80x9ccharacter projection,xe2x80x9d and xe2x80x9cblock exposurexe2x80x9d systems) have been devised. In each of the partial-pattern single-shot systems, certain circuit sub-patterns that are highly repeated in the layer being formed are repetitively transferred and exposed using an aperture mask on which one or more of the basic sub-patterns have been defined. An example of such a highly repeated sub-pattern is a memory cell dimensions of approximately 5-xcexcm square on the lithographic substrate (xe2x80x9csensitive substratexe2x80x9d). Unfortunately, with any of these techniques, variable-shaped-beam tracing is required to form on the substrate those portions of the pattern that are relatively non-repetitive. Consequently, overall throughput is too low for practical application for mass-production of wafers.
An attractive solution to the problem of substantially improving the throughput of CPB microlithography is the so-called xe2x80x9cone-shotxe2x80x9d pattern-transfer approach, in which the entire pattern for a layer in a single die (xe2x80x9cchipxe2x80x9d) or even for multiple dies is exposed in one xe2x80x9cshot.xe2x80x9d Unfortunately, this approach has not been realized from a practical standpoint for two main reasons. The first reason is that reticles suitable for exposing an entire die pattern in one shot are currently impossible to fabricate. The second reason is that CPB optical systems having optical fields sufficiently large for exposing an entire die pattern without significant off-axis aberrations are currently impossible to fabricate. Consequently, whereas the excitement over the potential of this approach remains high, engineering development work has been directed to other, more feasible, approaches.
One approach receiving much current attention involves dividing a reticle, defining a die pattern, into multiple portions usually termed xe2x80x9csubfields.xe2x80x9d Each subfield defines a respective portion of the overall pattern. The subfields are arrayed on the reticle in an ordered manner and are exposed in a sequential manner from the reticle to the substrate. This approach is termed the xe2x80x9cdivided reticlexe2x80x9d method, and apparatus configured for performing this method are termed xe2x80x9cdivided reticlexe2x80x9d projection-microlithography apparatus. By performing exposure subfield-by-subfield, the optical field can be sufficiently small to keep aberrations within specifications. Furthermore, any specific aberrations or other errors (e.g., distortion or errors in focus) that arise while exposing a particular subfield can be corrected, on the fly, in a manner that is most suitable for the particular subfield being exposed. The subfield images are placed contiguously on the substrate so as to form, after all the subfields are exposed, the complete die pattern on the substrate surface. Thus, overall exposure is performed with excellent resolution, accuracy, and precision across an optically much wider range than possible with the one-shot transfer method.
So as to expose various subfields on the reticle and form the respective images at proper locations on the substrate, a deflector is provided in the CPB optical system of the microlithography apparatus. This deflector imparts appropriate lateral deflections of the beam to reach the selected subfields. Conventionally, the deflector is an electromagnetic deflector. Since the subfields (which normally are arranged in a highly ordered array) are exposed sequentially by laterally deflecting the beam as required, the deflector experiences a predetermined, repetitive, energization sequence during exposure of the subfields. But, this ordered scheme of energizing the deflector must be stopped temporarily to allow use of the deflector in detecting alignment marks on the substrate by beam irradiation.
An electromagnetic deflector exhibits certain magnetic hysteresis characteristics that are best controlled when the deflector is being energized in a highly ordered manner. The reproducibility of its deflection characteristics is significantly lowered whenever the deflector is being energized in a non-ordered manner (e.g., for irradiating alignment marks). In addition, whenever the electromagnetic deflector is not being energized, the deflector temperature is reduced. Even a slight temperature reduction normally experienced after a shift from a highly ordered energization sequence (for exposing subfields) to a non-ordered energization (for exposing alignment marks) causes significant changes in the deflection characteristics exhibited by the deflector. These changes in deflection characteristics, in turn, cause corresponding errors in positional measurements performed using the beam, such as errors in detecting the positions of alignment marks relative to the axis of the optical system and/or the axes of the reticle stage and substrate stage. These errors generate subtle shifts in the subfield images as formed on the substrate, which decreases the accuracy of pattern transfer.
In view of the shortcomings of conventional methods and apparatus as summarized above, the invention provides, inter alia, charged-particle-beam (CPB) microlithography apparatus and methods that achieve more accurate detection of position-measurement marks than conventionally. I.e., detections of such marks are less influenced by temperature changes and hysteresis effects of deflection(s) used for performing such detection, which allows more accurate positional detections than currently achievable by current methods and apparatus.
To such ends and according to a first aspect of the invention, methods are provided, in the context of a CPB microlithography method, for detecting a position of a position-measurement mark situated within a deflection field of a CPB optical system. The CPB microlithography method is performed using a CPB optical system that is configured to projection-transfer respective images of exposure units of a pattern, defined on a divided reticle, to a sensitive substrate. In an embodiment of the subject method a charged particle beam (e.g., an electron beam) is deflected within the deflection field, according to a predetermined exposure sequence. While the charged particle beam is being deflected within the deflection field in a sequential manner, the position-measurement mark is irradiated with the charged particle beam. The position of the mark is detected at a moment in which the charged particle beam deflected according to the exposure sequence encounters the position-measurement mark in the deflection field.
The charged particle beam is deflected within the deflection field using a primary deflector in the CPB optical system. In this instance, the method can further comprise the step of scanning the charged particle beam over the position-measurement mark at the moment in which the charged particle beam encounters the position-measurement mark. Scanning of the charged particle beam over the position-measurement mark can be performed using a mark-scanning deflector separate from the primary deflector. After scanning the position-measurement mark, the mark-scanning deflector is de-activated, and sequential deflection is continued using the primary deflector.
The method can further comprise the step of moving the reticle and substrate as required for placing the position-measurement mark in the deflection field. In this instance at least one of the reticle and substrate can be moved to place the position-measurement mark on an optical axis of the CPB optical system. The position of the position-measurement mark is measured while the position-measurement mark is on the optical axis.
According to another aspect of the invention, methods are provided for detecting a relative position of the reticle and substrate, in the context of a CPB microlithography method. In an embodiment of such a method, respective position-measurement marks of the reticle and substrate are situated, within a deflection field of the CPB optical system, on the reticle and substrate. Using a primary deflector of the CPB optical system, a charged particle beam is deflected within the deflection field, according to a predetermined exposure sequence. While the charged particle beam is being deflected by the primary deflector within the deflection field in a sequential manner, the position-measurement marks are irradiated with the charged particle beam. The relative positions of the marks are detected at a moment in which the charged particle beam deflected by the primary deflector according to the exposure sequence encounters the position-measurement marks in the deflection field.
At the moment in which the charged particle beam encounters the position-measurement marks, a mark-scanning deflector in the CPB optical system can be actuated so as to scan the charged particle beam over the position-measurement mark on the substrate. After scanning the position-measurement marks, the mark-scanning deflector is de-actuated, and sequential deflection of the charged particle beam in the deflection field using the primary deflector is continued.
Desirably, the reticle and substrate are mounted on respective stages, in which instance the reticle stage and substrate stage are moved as required to place the position-measurement marks in the deflection field. The reticle stage and substrate stage can be moved to place the position-measurement marks on an optical axis of the CPB optical system, in which instance the relative positions of the position-measurement marks of the reticle and substrate desirably are measured while the position-measurement marks are on the optical axis.
In the methods summarized above, since mark detection is performed while the deflector is activated normally for exposure purposes, the temperature and hysteresis conditions of the deflector during mark detection can be substantially the same as during exposure. Thus, exposure accuracy and precision are improved. Desirably, the mark-scanning deflector has a relatively small deflection angle, and desirably only is operated at the moment, during normal deflection of the charged particle beam for exposure purposes, when the charged particle beam is to be used for mark detection. Also, the mark-scanning deflector exhibits low hysteresis.
In any of the methods summarized above, for example, if a weak detection signal is obtained, the relative positions of the marks can be detected multiple times to obtain multiple mark-detection signals. The mark-detection signals can be combined to obtain a stronger cumulative signal.
According to another aspect of the invention, CPB microlithography apparatus are provided. An embodiment of such an apparatus comprises a reticle stage, a substrate stage, and a CPB optical system. The reticle stage is configured to hold and move the reticle to place the subfields for illumination according to a predetermined exposure sequence. The substrate stage is configured to hold and move the sensitive substrate for imprinting of images of the illuminated subfields at respective locations on the sensitive substrate. The CPB optical system is situated relative to the reticle stage and has a deflection field. The CPB optical system is configured to direct a charged-particle illumination beam so as to illuminate a subfield on the reticle and to project a resulting charged-particle patterned beam, propagating downstream of the reticle, to form an image of the illuminated subfield on the sensitive substrate. The CPB optical system comprises a primary deflector and a mark-scanning deflector. The primary deflector is configured: (a) to direct the illumination beam so as to illuminate the subfields on the reticle sequentially within a deflection field, (b) to direct, as the subfields on the reticle are being illuminated, the resulting patterned beam so as to form images of the illuminated subfields at respective locations on the sensitive substrate within a corresponding deflection field, and (c) to direct the illumination and patterned beams as required to illuminate the respective position-measurement marks on the reticle and substrate. The mark-scanning deflector is configured to scan the patterned beam, which has passed through the respective position-measurement mark on the reticle, over the position-measurement mark on the sensitive substrate.
The CPB optical system desirably comprises an illumination-optical system situated upstream of the reticle and a projection-optical system situated between the reticle and the substrate. Each of the illumination-optical system and projection-optical system comprises a respective primary deflector. Desirably, each of the primary deflectors is an electromagnetic deflector, and the mark-scanning deflector is an electrostatic deflector or small hollow-core electromagnetic deflector.
The illumination-optical system and projection-optical system have respective deflection fields. The primary deflector of the illumination-optical system is configured to deflect the illumination beam within the deflection field of the illumination-optical system, and the primary deflector of the projection-optical system is configured to deflect the patterned beam within the deflection field of the projection-optical system. The mark-scanning deflector desirably is located in the projection-optical system, wherein the mark-scanning deflector is configured to scan the patterned beam, as the illumination beam is illuminating the position-measurement mark on the reticle, over the respective position-measurement mark on the substrate.
The length of the deflection field of the illumination-optical system corresponds to a length of an electrical stripe of subfields arranged on the reticle. Similarly, the length of the deflection field of the projection-optical system corresponds to a length of an electrical stripe of subfields as projected onto the sensitive substrate.
Typically, the CPB optical system has an optical axis. In such a configuration, the reticle stage can be configured to place the position-measurement mark, on the reticle, on the optical axis as the illumination beam illuminates the position-measurement mark. Similarly, the substrate stage can be configured to place the position-measurement mark, on the substrate, on the optical axis as the patterned beam from the illuminated position-measurement mark on the reticle impinges on the position-measurement mark on the substrate. The mark-scanning deflector can be configured to scan the patterned beam, as the illumination beam is illuminating the position-measurement mark on the reticle, over the respective position-measurement mark on the substrate.
The apparatus can further comprise a detector situated and configured to detect backscattered charged particles from the position-measurement mark on the sensitive substrate.
The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.